Plasma biomarkers, memory impairment, and imaging measures of Alzheimer’s disease in American Indians

Abstract

AbstractBackgroundAlzheimer’s disease (AD) is best defined in vivo using quantifiable, objective markers reflecting amyloid deposition (A), pathologic tau (T), and neurodegeneration (N). However, gold standards for ATN (imaging or cerebrospinal proteins) are expensive, invasive, and/or impractical in low resource settings. Circulating proteins offer a practical alternative. Candidate plasma markers include amyloid‐β (Aβ40&42), phosphorylated tau‐181 (ptau), glial fibrillary protein (GFAP), and neurofilament light chain (NfL). However, no such markers have been evaluated in American Indians.MethodThe Strong Heart Study recruited 401 American Indians over 3 states in 2017‐2019 for MRI, cognitive testing, and clinical examination. MRI included T1 sequences, which were processed using FreeSurfer to quantify structural volumes. Neuropsychological testing incuded California Verbal Learning Test, which was operationalized into categories of memory impairment using indices of learning and recall. Plasma samples were sent to University of Gothenburg for Simoa Quanterix assay. Statistics included descriptive summary; multivariate regression; and receiver operating characteristic analysis.ResultThe study population was mean age 78 (SD 4.7, range 70‐95); majority female (71%); and 21% APOE ε4 allele carriers. An estimated 31% had any memory impairment (13.8% retrieval, 6.5% storage, 10.7% encoding). Median plasma marker values were: ptau 5.0pg/mL (range: 1.5‐442), Aβ40 128pg/mL (116‐175), Aβ42 8.2pg/mL (6.7‐9.7), GFAP 150pg/mL (0‐651), and NfL 31.6pg/mL (9.7‐343); median Aβ42:40 ratio 0.06. Comparing APOEε4 carriers to non‐carriers (Table1), ptau was +12%, Aβ40 ‐0.8%, Aβ42 ‐8.4%, Aβ4240 ‐16.7% (P = 0.006), GFAP +16.1%, NfL +3.2%, but only Aβ4240 was significant. Linear regressions with brain volumes identified significant associations for Aβ42, Aβ40, GFAP, and NfL (but not ptau) with ventricle, cortical, and overall volume; NfL was also associated with entorhinal and hippocampal volume. GFAP and ptau were significantly higher in those with memory encoding deficit; NfL higher with encoding or retrieval deficit (Table2). Although not significant, Aβ40 was lower with storage deficit, and Aβ42 with encoding or storage deficit. AUC range for plasma markers to discriminate memory impaired from intact: 0.4‐0.6.ConclusionThese data are a first look at ATN plasma markers in a population‐based cohort of American Indians, an underrepresented group in AD research. Future research should address generalizability and measurement validity.